The invention relates to the formation of an exfoliated or intercalated polymer-clay nanocomposite composition comprising a star-shaped polymer, and methods of forming such a composition.
There have been extensive efforts directed towards the preparation of various polymer-clay composite materials since the discovery of exfoliated nylon/clay nanocomposites by Usuki et al. (J. Mater. Res. 1993, 8, 1174). Such materials are expected to have new and improved properties compared to those of the polymers alone. Such improved properties include mechanical, thermal, and barrier properties. For example, see M. Alexandre and P. Dubois Mater. Sci. Eng. 2000, 28, 1; and T. J. Pinnavaia and G. W. Beall Polymer-Clay Nanocomposites, John Wiley and Sons, Ltd. New York, 2000, pp. 195-226.
Incorporation of a few percent of clay has been found to greatly increase a polymer""s modulus, strength, gas barrier properties, and heat distortion temperature. The presence of clay is also reported to impart fire retardant properties. Without wishing to be bound by theory, the improvement in thermal stability is believed to be attributed to tortuous diffusion of the volatile thermal and thermo-oxidative decomposition products in the presence of dispersed silicate layers. The slower diffusion of decomposed volatiles along with structural reinforcement provided by the char formed with collapsing silicate layers during combustion contributes to decreased flammability of exfoliated nanocomposite as demonstrated by cone calorimetry. See for example, J. Zhu and C. A. Wilkie Polym. Int. 2000, 49, 1158; and J. W. Gilman Appl. Clay Sci. 1999, 15, 31.
The most common morphology for miscible polymer-clay dispersions is known as intercalation. In this case, the host polymer penetrates the space between the clay platelets, but separating them only slightly and maintaining the parallel, regular structure of the platelets. Intercalated polymer-clay nanocomposites are often observed to have measurable improvements in physical properties, but typically less so than if the corresponding nanocomposite were in the morphology known as exfoliation. Although exfoliation is much more desirable, it is less common and more difficult to obtain. In this morphology, the clay platelets are thoroughly separated from each other by the host polymer, so that their original crystallographic register is lost. Particularly for nonpolar polymer hosts, the fully exfoliated polymer-clay nanocomposites are notoriously difficult to obtain.
Non-polar polymers, including polystyrene and polyethylene, represent a group of commercially important thermoplastics. For example, more than five billion pounds of polystyrene are produced annually in the US and injection molded or extruded into specific products. Polystyrene and linear low density polyethylene (LLDPE) homopolymers form intercalate morphologies when melt blended with organically modified clays known as organoclays. See, for example, R. A. Vaia and E. P. Giannelis Macromolecules 1997, 30, 8000; and also K. H. Wang et al. Polymer, 2001, 42, 9819.
Although exfoliation of clay in these non-polar polymers is more desirable, achieving this state of morphology is particularly challenging because the polymers are not strongly attracted to the clay surfaces. One approach to achieve this goal is in-situ polymerization of non-polar monomers in the presence of organoclay bearing either polymerizable functional groups or initiators. See, for example, X. Fu and S. Qutubuddin Polymer 2001, 42, 807; J. Zhu et al, Chem. Mater. 2001, 13, 3774; and M. W. Weimer et al. J. Am. Chem. Soc. 1999, 121, 1615.
Another method to achieve exfoliation of such non-polar polymers is to incorporate polar units in the backbone of non-polar guest polymers and subsequently melt blend them with a host organoclay. See, for example, N. Hasegawa et al. J. Appl. Polym. Sci. 1999, 74, 3359; and C. I. Park et al. Polymer 2001, 42, 7465. The latter strategy is industrially more feasible, but increasing the fraction of polar units in non-polar polymers may result in undesirable morphological changes, e.g., phase separation. Thus, complete exfoliation of organoclay in non-polar homopolymer has not been commercially or economically practicable on a comparable scale.
The phase behavior of polymer/clay composites has been the subject of recent computational studies. See A. C. Balazs et al. Acc. Chem. Res. 1999, 32, 651; and C. Singh and A. C. Balazs, Polym. Int. 2000, 49, 469. The thermodynamic models of Balazs and Singh theoretically predict a better intercalation and, in some cases, when the interactions between the organic modifiers and the polymer chains is enhanced, exfoliation of the organoclay in pure star-shaped polymers.
There is a need for an exfoliated polymer-clay nanocomposite composition comprising clay and polystyrene (a very non-polar polymer) which composition can be produced in large scale volume by a commercially practical method.
Applicants have discovered that an exfoliated or intercalated nanocomposite of clay with polystyrene, including polymer blends therewith, can be prepared by first compounding a special polymer comprising polystyrene with the clay. Compounding in this case means simply heating a physical mixture of the special polymer and clay above the glass transition temperature of the special polymer. The special polymer has a star architecture and is referred to as a xe2x80x9cstar polymer.xe2x80x9d A star polymer is made of several (at least three) branches radiating from a central core. In contrast, normal linear polystyrene does not form an exfoliated nanocomposite with clay, even after prolonged heating. A nanocomposite made with star-shaped polystyrene can be blended with normal linear polystyrene, yet still maintain an exfoliated morphology.
The present invention covers clay-polymer nanocomposites in either exfoliated form or, for use in making exfoliated nanocomposites upon further processing, the clay-polymer nanocomposites in intercalated form. The nanocomposites can be blended with other polymers or additional amounts of polymer to obtain a desired composition. A mixture of clay, star polymer, and linear polystyrene, whether in intercalated or exfoliated form, will be referred to herein as a xe2x80x9cthree-partxe2x80x9d mixture, and a mixture of clay and star polymer, but without linear polystyrene, whether in intercalated or exfoliated form, will be referred to as a xe2x80x9ctwo-part mixture,xe2x80x9d although additional components may be present in a mixture. Either a two-part mixture or a three-part mixture may be formed initially, and higher amounts of linear polystyrene added to achieve the desired composition. Once exfoliation of the mixture is achieved, the exfoliated structure can be maintained even in the presence of additional or higher amounts of linear polystyrene.
The present invention can be used to provide an exfoliated polystyrene-clay material with greatly improved physical properties in several respects, compared to the polymer, even with a very low content of clay. For example, the modulus, fracture toughness, gas barrier properties, and heat distortion temperature of such materials can be increased significantly. In addition, improved fire retardant properties can be imparted to the nanocomposite by the presence of the clay in the polymer.
The invention is also directed to a process that can be used to make compositions of the present invention. In one embodiment, a physical mixture comprising powdered clay and star polymer is prepared, and then the mixture is heated for a sufficient period of time, preferably under high-shear mixing, to increase the rate of exfoliation.
As indicated above, only a small amount of clay (1-10% by weight) is necessary to significantly improve the physical properties of the polystyrene-containing composition. Similarly, only a relatively small amount of the star polymer is required. In a preferred embodiment, therefore, a final nanocomposite can be made comprised largely of relatively inexpensive commodity linear polystyrene polymer.